From DE 10 2005 006 404 B3, an In-The-Ear hearing device is known, which can temporarily be complemented by a supplementary module attachable to a hearing device user's head. The In-The-Ear hearing device has all components and functionalities required for operation, whereas the supplementary module can change or add functionalities.
In U.S. Pat. No. 5,987,146, an open ear canal hearing aid system is disclosed which comprises a plurality of ear canal tubes sized for positioning in an ear canal of a user so that the ear canal is at least partially open for directly receiving ambient sounds. The hearing aid system comprises a case to which the ear canal tubes are connected and which is designed to fit behind a user's ear. In said case, a sound processor is arranged for amplifying ambient sounds received through one of the ear canal tubes and for producing processed sounds and for supplying the processed sounds to the second ear canal tube. A speaker is arranged within said case, and a microphone is arranged at that end of one of the ear canal tubes which is located in the ear canal.
In WO 2008/010716 A2, a hearing-aid device is presented, which comprises a device housing intended and adapted to be worn outside an ear of a user and which co-acts with an in-the-ear part. The in-the-ear part is physically separated from the device housing but electronically connected thereto and comprises a microphone and a loudspeaker. The device housing or the in-the-ear part comprises a digital processing device in which sound picked up by the microphone is processed, and the processed sound is fed to the loudspeaker.